SEDIMENTARY TRENDS IN THE LOWER AND UPPER BALD EAGLE SANDSTONE, MILROY, PA

Petrographic and petrologic studies of the Upper Ordovician, Bald Eagle Sandstone at Milroy, Pennsylvania have been the focal point of the study. The 510 foot-thick formation contains the lower (Centennial School) and upper (Lost Run) members, with the boundary between these at 192 feet at this particular outcrop. The lower member contains generally fine-grained sandstone with interbedded shale and siltstone, while the upper member consists of coarser grained sandstone and interbedded conglomerate. We sampled at ten-foot intervals from the lower 480 feet of section and performed 300-grain point counts for these samples. The overall section shows a gradual coarsening upward with quartz being the main constituent. Grains are well-rounded suggesting significant transport.

Sandstone from this section ranges from quartz arenite to sublitharenite. Using Dickinson plots, all samples plot as quartzose recycled orogen. Undulatory quartz is most abundant (typically >50%) while nonundulatory and polycrystalline quartz are also main constituents (~25%). In general, rock becomes less quartzose and more lithic-rich in the lower 120 feet of the section. The 0 to 70 foot interval contains mature sandstone, comprised of 87-93% quartz. Between 70 to 120 foot, quartz content ranges between 82-89%. A significant lithic-rich interval falls between 120 and 230 feet in the section with lithic concentrations of 12-17%, and quartz concentrations as low as 76%. This interval encompasses the lower/upper member boundary, with no significant change in lithology at the boundary. Above 230 feet in the section, the sandstone is more quartzose approaching 90% quartz. It is presently unclear if this mid-Bald Eagle lithic pulse is related to uplift in the source area, climatic influences, or increases in sedimentary accommodation space. However, it is a lithostratigraphic signature that can be investigated at other exposures of the Bald Eagle.